1. Field of the Invention
The invention relates to a four-stroke Otto engine having at least one cylinder and, for each cylinder, at least one intake valve with positive control and at least one outlet valve with positive control, with at least one intake conduit for the supply of air or an air-fuel mixture to the intake valve.
2. Description of the Prior Art
Because of operating conditions and the broad range of areas of application, contemporary four-stroke Otto engines are designed for a large rpm/load range. To realize high torque and output data, and an acceptable efficiency and exhaust-gas emission ratio in partial-load operation and/or in the lower rpm range, compromises in design must be made with regard to the design of the valve-control time, the valve stroke, the valve seat geometry, the structure of the intake conduit and intake system, and the mixture formation. The primary operating range of the engines lies within the lower rpm/load range, i.e., at an effective medium pressure of 0 to approximately 5 bar and at an rpm between the idling rpm and approximately 3500 l /min. This operating range covers the dwell period in the test cycles prescribed for type and series/model standard for determining exhaust gas emissions and fuel consumption up to above 90 percent.
In four-stroke Otto engines, quantity-controlled output control is regulated by the mass of working fluid used. In a camshaft-controlled load change, a throttling of the fresh intake mass is necessary in order to be able to operate the engine in all operating ranges, that is, from idling operation with a low fresh intake mass to full-load operation with a fully open throttle element. In these throttle-controlled engines, an adaptation of the charge composition (residual gas, fresh intake mass) and an rpm-dependent adaptation of the valve control times to full-load operation is typically effected solely by a phase shift of the valve control times.
Throttle-free control of the fresh intake mass used is not possible in the entire operating range with this phase shift. For operation at higher rpms and loads, the intake valve cross-section must be enlarged for the charge change. This is realized in a practical manner by the connection of one or a plurality of valves. These connectable valves can be operated by means of a camshaft, in which case the charge quantity is throttle-controlled. Stepless regulation and control of the charge quantity and the charge composition can be accomplished with fully-variable intake valve control times. The charge change losses occurring in internal combustion engines, depending on their operating principles, can thus be eliminated. Furthermore, an adaptation of the charge composition is possible in all operating ranges.
The disadvantages of throttle-controlled combustion engines occur predominantly in the lower rpm/load range. If this operating range is used with systems for realizing fully-variable control times, the fuel consumption behavior and exhaust gas emissions are improved through saving throttle losses and the adaptation of the charge composition. Systems for realizing completely-variable control times, especially when actuation is effected by electromechanical setting units, can be designed especially for this lower operating range in the characteristic field with respect to optimum efficiency.